Can't spawn riding mob via Command

Riding Dragon to a Giant or Any mods can't spawn

I expect that my command would perfectly working . It will spawn a Dragon riding a giant or Giant riding a dragon .

It spawned and dissappeared.

Steps to Reproduce:
1. I type this command "summon Giant ~ ~ ~ {Riding:{id:Dragon}}" , or this "summon EnderDragon ~ ~1 ~ {Riding:{id:Giant}}"
2. Then I press it and it spawn the mob then despawn again . Sometimes it cause to my game to crash .